1. Field of the Invention
The present invention relates generally to power supply circuits for transferring energy from a first windint to second and third windings of a transformer for charging a battery when AC power is normal and transferring energy of the battery from the second winding to a third winding when the AC power fails.
2. Description of the Related Art
In a prior art uninterrupted power supply unit disclosed in Japanese Laid-Open Patent Specification Hei-3-98431, a first capacitor which is normally charged with DC energy is repeatedly discharged at high frequency into the first winding of a transformer to induce a high-frequency current in a secondary winding, where it is rectified to charge a second capacitor as well as in a backup winding. A battery connected to the backup winding is charged by a rectified current of the backup winding. When the mains supply power fails, the battery is repeatedly discharged into the backup winding to induce a high frequency current in the secondary winding. To perform switching from the normal mode to the battery discharge mode, a power failure detector and a mechanical switching circuit are provided. When the power fails, the detector operates the switching circuit to deactivate the first winding and activate the backup winding to produce high-frequency oscillations.
However, the use of a mechanical switch is unfavorable from the manufacturing point of view because of the recent tendency toward using all electronic circuitry. Furthermore, the power failure detector represents an extra cost and added complexity.